1. Field of the Invention
The present invention relates to a method of producing a planographic printing plate. In particular, the present invention relates to a method of producing a planographic printing plate, in which a positive-working planographic printing plate precursor for an infrared laser for direct plate making, which allows direct plate making from a digital signal from computers and the like, is used.
2. Description of the Related Art
Recently, significant progress has been made in the development of lasers, and in particular, solid lasers and semiconductor lasers having high output and small size, and also a light emission range in the near-infrared to infrared region, are widely available. At present, various light sensitive compositions are used as a visible image forming material or a material for recording a planographic printing plate precursor, and the aforementioned lasers are very useful as a light source for light exposure of a light sensitive composition to directly form an image from digital data such as computer data.
A positive-working planographic printing plate precursor for an infrared laser has an alkali soluble binder resin, an infrared absorbing agent that absorbs light and generates heat (that is, an IR dye), and the like, as essential components. In an unexposed region (that is, an image area), such an infrared absorbing agent or the like functions as a dissolution inhibiting agent which substantially decreases the solubility of a binder resin, owing to the interaction between the IR dye and the binder resin. Meanwhile, in an exposed region (that is, a non-image area), the interaction between the IR dye and the binder resin is suppressed by the heat generated in the exposed region, whereby the exposed region dissolves in an alkaline developer and a planographic printing plate is formed.
However, in a positive-working planographic printing plate precursor for an infrared laser, a difference (that is, solubility discrimination: hereinafter, referred to as “solubility discrimination” as appropriate) between the dissolution resistance of a region unexposed (that is, an image area) to a developer and the dissolution property of an exposed region (that is, a non-image area) is not sufficient under various use conditions, and problems like over-development and imperfect development may easily occur as a result of a change in use conditions. Further, even in a case in which the surface state is slightly changed by contact with the surface during handling or the like, there has been a problem that the unexposed region (that is, the image area) is dissolved during development to form a scratch shape, yielding insufficient receptivity or reduced scratch resistance caused by a defect in the image area.
A compound represented by an infrared absorbing agent that functions as a dissolution inhibiting agent in the recording layer of a positive-working planographic printing plate precursor for an infrared laser works only as a dissolution inhibiting agent for an unexposed region (image area), and does not promote the dissolution of an exposed region (non-image area). Therefore, in order to provide a difference in dissolution property between an unexposed region and an exposed region in a positive-working planographic printing plate precursor for an infrared laser, it is necessary to use in advance, as a binder resin, one with high solubility in an alkali developer. As a result, the state before development becomes unstable. Further, in a planographic printing plate precursor, an ink-receiving recording layer is formed on a hydrophilic support. As a result, there is a problem that the adhesion property of the recording layer is unstable at an interface with a support and the scratch resistance of an unexposed region (image area) is also affected thereby. Therefore, from the viewpoint of image reproducibility and also recent demands for high resolution images, sufficient dissolution discrimination is now demanded so that the durability of an image area can be improved in a formed planographic printing plate.
To solve the problems described above, various suggestions have been made. For example, for the purpose of improving wear resistance or development resistance, a method of having an alkali soluble polymer compound having a partial siloxane structure or an alkali soluble polymer compound having a fluoroalkyl group and localizing the resin near the surface of an image recording layer has been suggested (for example, see Japanese Patent Application Laid-Open (JP-A) Nos. 2005-181963 and 2006-53487).